Application of polyoxotungstates as environmental catalysts: wet air oxidation of acid dye Orange II

The catalytic effect of two Keggin-type polyoxotungstates, namely H4SiW12O40 and Na2HPW12O40, on the wet air oxidation of azo dye Acid Orange 7 (AO7; C-dye = 248 muM) at varying reaction temperatures (T = 160-290 degreesC at PH2O = 0.6-3.0 MPa, respectively) was examined. Arrhenius parameters for catalytic and non-catalytic wet air oxidation were obtained in terms of temperature dependent AO7 and TOC abatement kinetics. To get a further insight into the dominant reaction pathway of the catalytic and non-catalytic wet air oxidation processes, isopropyl alcohol and bromide (300 muM) were added as OH. scavengers. Obtained results revealed an important homogenous contribution for WAO of AO7 (Arrhenius parameters E-a = 84 kJ mol(-1) and A = 5.6 x 10(7) min(-1)). However, mineralization (TOC removal) was noticeably influenced by the investigated polyoxotungstates (C-catalyst = 140 muM) and the activation energy decreased from 40 kJ mol(-1) in the absence of catalyst to only 28 U mol(-1) (PW12 (3-)) and 22 kJ mol(-1) (SiW124-). The catalytic processes were appreciably less sensitive to the presence of OH. scavengers than uncatalyzed wet air oxidation implying that polyoxotungstate-catalyzed wet air oxidation did not solely depend upon a free radical type reaction mechanism. The obtained results tend to support the free radical chain mechanism to account for Orange dye degradation only for uncatalyzed wet air oxidation. (C) 2002 Elsevier Science Ltd. All rights reserved.